CCLM-based intra-prediction method and device

1. A method for decoding a picture, performed by a decoding apparatus, the method comprising:

deriving neighboring chroma reference samples for a chroma block;

deriving neighboring luma reference samples of a luma block related to the chroma block and luma samples in the luma block;

deriving down-sampled neighboring luma reference samples and down-sampled luma samples by down-sampling the neighboring luma reference samples and the luma samples;

deriving a linear model parameter based on the down-sampled neighboring luma reference samples and the neighboring chroma reference samples;

generating prediction samples for the chroma block based on the linear model parameter and the down-sampled luma samples of the luma block; and

reconstructing the chroma block based on the prediction samples for the chroma block,

wherein the neighboring luma reference samples includes top neighboring luma reference samples located at the upper side of a top boundary of the luma block, and left neighboring luma reference samples located at the left side of a left boundary of the luma block,

wherein based on the luma block located in a current coding tree unit (CTU) that is different from a CTU in which the top neighboring luma reference samples is located, the number of the top neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples among the neighboring luma reference samples is less than that of the left neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples, and

wherein based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the top neighboring luma reference samples are located in a horizontal 1 sample line adjacent to the top boundary of the luma block and the left neighboring luma reference samples are located in at least vertical 2 sample lines neighboring the left boundary of the luma block.

2. The method of claim 1 , wherein the down-sampled neighboring luma reference samples comprise down-sampled left neighboring luma reference samples and down-sampled top neighboring luma reference samples, and

based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the number of the top neighboring luma reference samples used for deriving one down-sampled top neighboring luma reference sample is three, and the number of the left neighboring luma reference samples used for deriving one down-sampled left neighboring luma reference sample is six.

3. The method of claim 2 , wherein the three top neighboring luma reference samples are located at coordinates (2*x−1, −1), (2*x, −1), and (2*x+1, −1), respectively, when the down-sampled top neighboring luma reference sample has a coordinate (x, −1).

4. The method of claim 2 , wherein the three top neighboring luma reference samples are located at coordinates (−1, −1), (0, −1) and (1, −1), respectively, when the down-sampled top neighboring luma reference sample has a coordinate (0, −1) and there is a sample value of a luma reference sample corresponding to the coordinate (−1, −1).

5. The method of claim 1 , wherein the linear model parameter comprises a first linear model parameter representing a scaling factor and a second linear model parameter representing offset.

6. A method for encoding a picture, performed by an encoding apparatus, the method comprising:

deriving neighboring chroma reference samples for a chroma block;

deriving neighboring luma reference samples of a luma block related to the chroma block and luma samples in the luma block;

deriving down-sampled neighboring luma reference samples and down-sampled luma samples by down-sampling the neighboring luma reference samples and the luma samples;

deriving a linear model parameter based on the down-sampled neighboring luma reference samples and the neighboring chroma reference samples;

generating prediction samples for the chroma block based on the linear model parameter and the down-sampled luma samples of the luma block;

deriving residual samples for the chroma block based on the prediction samples for the chroma block; and

encoding picture information including information for the residual samples,

wherein the neighboring luma reference samples includes top neighboring luma reference samples located at the upper side of a top boundary of the luma block, and left neighboring luma reference samples located at the left side of a left boundary of the luma block,

wherein based on the luma block located in a current coding tree unit (CTU) that is different from a CTU in which the top neighboring luma reference samples is located, the number of the top neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples among the neighboring luma reference samples is less than that of the left neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples, and

wherein based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the top neighboring luma reference samples are located in a horizontal 1 sample line adjacent to the top boundary of the luma block and the left neighboring luma reference samples are located in at least vertical 2 sample lines neighboring the left boundary of the luma block.

7. The method of claim 6 , wherein the down-sampled neighboring luma reference samples comprise down-sampled left neighboring luma reference samples and down-sampled top neighboring luma reference samples, and

based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the number of the top neighboring luma reference samples used for deriving one down-sampled top neighboring luma reference sample is three, and the number of the left neighboring luma reference samples used for deriving one down-sampled left neighboring luma reference sample is six.

8. The method of claim 7 , wherein the three top neighboring luma reference samples are located at coordinates (2*x−1, −1), (2*x, −1), and (2*x+1, −1), respectively, when the down-sampled top neighboring luma reference sample has a coordinate (x, −1).

9. The method of claim 7 , wherein the three top neighboring luma reference samples are located at coordinates (−1, −1), (0, −1) and (1, −1), respectively, when the down-sampled top neighboring luma reference sample has a coordinate (0, −1) and there is a sample value of a luma reference sample corresponding to the coordinate (−1, −1).

10. The method of claim 7 , wherein, when the down-sampled top neighboring luma reference sample has a coordinate (0, −1) and there is no sample value of the luma reference sample corresponding to a coordinate (−1, −1), in order to derive the down-sampled top neighboring luma reference sample, a one top neighboring luma reference sample is used, and a sample value of the down-sampled top neighboring luma reference sample is determined as a sample value of the top neighboring luma reference sample located at the coordinate (0, −1).

11. The method of claim 6 , wherein the linear model parameter comprises a first linear model parameter representing a scaling factor and a second linear model parameter representing offset.

12. A non-transitory computer-readable storage medium storing encoded picture information generated by a method for encoding a picture, the method comprising:

deriving neighboring chroma reference samples for a chroma block;

deriving neighboring luma reference samples of a luma block related to the chroma block and luma samples in the luma block;

deriving down-sampled neighboring luma reference samples and down-sampled luma samples by down-sampling the neighboring luma reference samples and the luma samples;

deriving a linear model parameter based on the down-sampled neighboring luma reference samples and the neighboring chroma reference samples;

generating prediction samples for the chroma block based on the linear model parameter and the down-sampled luma samples of the luma block;

deriving residual samples for the chroma block based on the prediction samples for the chroma block; and

encoding picture information including information for the residual samples,

wherein the neighboring luma reference samples includes top neighboring luma reference samples located at the upper side of a top boundary of the luma block, and left neighboring luma reference samples located at the left side of a left boundary of the luma block,

wherein based on the luma block located in a current coding tree unit (CTU) that is different from a CTU in which the top neighboring luma reference samples is located, the number of the top neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples among the neighboring luma reference samples is less than that of the left neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples, and

wherein based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the top neighboring luma reference samples are located in a horizontal 1 sample line adjacent to the top boundary of the luma block and the left neighboring luma reference samples are located in at least vertical 2 sample lines neighboring the left boundary of the luma block.

13. The method of claim 1 , wherein based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the top neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples are located in the horizontal 1 sample line adjacent to the top boundary of the luma block and the left neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples are located in vertical 3 sample lines neighboring the left boundary of the luma block.

14. The method of claim 6 , wherein based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the top neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples are located in the horizontal 1 sample line adjacent to the top boundary of the luma block and the left neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples are located in vertical 3 sample lines neighboring the left boundary of the luma block.

15. The non-transitory computer-readable storage medium of claim 12 , wherein based on the luma block located in the current CTU that is different from the CTU in which the top neighboring luma reference samples is located, the top neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples are located in the horizontal 1 sample line adjacent to the top boundary of the luma block and the left neighboring luma reference samples used for deriving the down-sampled neighboring luma reference samples are located in vertical 3 sample lines neighboring the left boundary of the luma block.